The present invention generally relates to the field of mechanisms which modify the displacement or force generated by an actuator. More specifically, the present invention relates to a core structure, and devices having a plurality of these core structures, that relies on the elastic deformation of its constituent elements to transmit forces and motion from an input to an output. In one specific embodiment, the preset invention relates to the field of microelectromechanical (MEM) systems and, in particular, to a pivotless structure formed by surface micromachining processes for use in combination with a MEM actuator (such as an electrostatic comb actuator, a capacitive-plate electrostatic actuator) or a thermal actuator to modify a displacement or force provided by the MEM actuator.
Historically, engineered devices are designed to be strong and stiff and engineered systems are usually assembled from discrete components. In nature, however, designs are strong and compliant and natural systems develop as one connected whole.
The limitation of many currently available actuators, and in particular smart material actuators, is their small stroke and power. Currently, piezoelectric actuators have a high bandwidth, but low strain, whereas shape memory alloy (SMA) actuators have relatively high strain, but extremely low bandwidth. The use of the above-type actuators in a system design must therefor involve a stain versus weight versus bandwidth tradeoff. In an effort to amplify their displacement, some investigators have pursued the stacking of multiple actuators in different configurations. Although modest displacement amplification can be achieved in this manner, these arrangements are often cumbersome and impose a heavy penalty by way of voltage requirements.
In the formation of many types of microelectromechanical (MEM) devices, a motive forcer or actuator is required. The previous used electrostatic comb actuators have generally consumed a large portion of the die on which the MEM device is formed (e.g. up to ⅔ of the die size). Further, the die size is constrained by available steppers used for the photolighographic processes of the MEM fabrication process. As a result, the size and complexity of MEM devices is presently constrained by the size of the actuator used.
Reducing the size of the motive source can alleviate this problem and leave more space on the die for MEM devices of increased complexity and functionality. A smaller-sized electrostatic comb actuator, however, will produce a correspondingly smaller range of displacement (i.e. a smaller actuation stroke) which can be, for example, about 5 microns or less. Thermal actuators and capacitive-plate electrostatic actuators generally provide a much larger force than is available with electrostatic comb actuators. However, this larger force is generated over a short actuation stroke of typically 0.25-2 xcexcm. Such a short actuation stroke is insufficient for driving many types of MEM devices including ratchet-driven gears, stages or racks; or microengines such as that disclosed in U.S. Pat. No. 5,631,514 to Garcia et al.
From the above it is seen that there is a need in a MEM device for a mechanism that multiplies the range of displacement from a short-stroke actuator and provides an increased range of displacement that is sufficient for actuating a particular MEM device. This will allow the use of compact electrostatic comb actuators or, alternately, capacitive-plate or thermal actuators, allowing the formation of MEM devices of increased complexity and functionality within a give die size.
Employing mechanical means to modify (amplify or attenuate or reorient) an output displacement or force is not new. Perhaps the simplest displacement-amplifying device is a lever arm moving about a pivot joint. A lever arm is shown in FIG. 1. The use of a pivoted displacement-modifying device, however, may be undesirable in certain situations. For example, if a linear output is desired, devices utilizing a pivoting lever arm provide movement d2 at the output end of the lever arm which is arcuate. Play in the pivot joint, which is limited by fabrication tolerances, can render a lever arm undesirable, as can a required high geometric advantage, which would require extreme length in L2. The play in the pivot joint of a MEM device can be substantial compared to the range of displacement of a short-stroke actuator. For example, a thermal actuator can provide a range of displacement that is only 0.25 xcexcm, when heated from room temperature to about 400xc2x0 C. This is comparable to the actual amount of play in MEM pivot joints and as such the use of a displacement-multiplying device having a pivot joint would not be suitable for use in increasing the range of displacement of a thermal actuator. Additionally, an arcuate output displacement can complicate the design of the MEM device.
Another variety of a displacement modifying strategy is a rigid link system. FIG. 2 illustrates a crimping mechanism employing a conventional rigid link displacement modifying strategy. When operated in one direction the mechanism amplifies displacement and when operated in the opposing direction it amplifies force.
Compliant mechanisms are a relatively new class of devices that utilize elasticity or compliance elements to transmit motion and/or force. They can be designed for any desired input-output force displacement characteristics, including specified volume/weight, stiffness and natural frequency constraints. A xe2x80x9ccompliant mechanismxe2x80x9d is defined herein as a structure that exploits elastic deformation to achieve a force/displacement transformation, with the displacement being changed (e.g. increased) relative to an input force provided to the same end of the compliant structure. In one direction of operation, an output displacement is increased relative to an input displacement; and an output force is correspondingly decreased relative to an input force. In the other direction of operation, the opposite effect occurs with the output force being increased relative to the input force; and with the output displacement being decreased relative to the input displacement. As flexure is permitted in these mechanisms, they can be readily integrated with unconventional actuation schemes including the above mentioned actuators. Compliant mechanisms can be made from any ductile material such as nylon, aluminum, steel, nickel-titanium alloy, etc.
More specifically, compliant mechanisms are single-piece flexible structures that deliver the desired motion by undergoing elastic deformation as opposed to rigid body motions of conventional mechanisms. As compliant mechanisms are thus far known in the art, they have been limited to the replacement of mechanical hinges with flexural joints (living hinges). One such mechanism having a living hinge flexural joint is illustrated in FIG. 3. However, simply replacing mechanical joints with flexural joints has the disadvantages of being a source of stress concentration leading to early fatigue failure (as a result of the flexural joints being unusually thin and resultant stress concentrations), being a source of significant efficiency loss due to the large localized strain energy loss at each of the flexural joints and can be difficult to manufacture.
Compliant mechanisms having flexural joints are seen to exhibit what is herein referred to as xe2x80x9clumped compliancexe2x80x9d. Lumped compliance results because the thin flexural joints localize strain energy losses where flexing in the device occurs, at each of the flexural joints. Between the flexural joints, these structures generally operate in a rigid manner. FIG. 3b illustrates in cross-section a lumped compliant device as might be used in a precision instrument.
The present invention proposes a deviation from the known art of compliant mechanisms. More specifically, the present invention proposes a compliant mechanism which lacks the flexural joints of the prior art. A compliant mechanism according to the present invention is pivotless or jointless and exhibits what is herein referred to as xe2x80x9cdistributed compliancexe2x80x9d. In other words, compliance spread across the mechanism""s elements themselves, whereby energy is transferred efficiently from the input actuator to the output.
It is therefore an object of the present invention to provide a pivotless compliant mechanism to modify the displacement of force generated by an actuator.
A further object of this invention is to provide a single-piece pivotless compliant mechanism that relies on an elastic deformation distributed across its constituent elements to transmit forces and displacement from the input to the output.
A related object of this invention is to provide a pivotless compliant mechanism where the joining of two of its constituent members exhibits a more or less uniform cross-section and not a thin flexural joint.
An advantage of this invention is that the mechanism is very energy efficient, easy to manufacture, long lasting and highly reliable.
Still another object of this invention is to provide a pivotless compliant mechanism that can be used in combination with a MEM actuator to form a MEM apparatus having a different output displacement and force from that provided by the actuator.
An advantage of the present invention is that play due to fabrication tolerances is substantially reduced compared with pivoting mechanisms.
Yet another advantage of the present invention is that a linear displacement from a MEM actuator can be provided as an input to the pivotless compliant mechanism to generate a different displacement range that is also linear (i.e. along a straight line).
A further advantage of the present invention is that the pivotless compliant mechanism can be designed to respond to an input force and displacement and generate therefrom an output force and displacement that is directed either substantially in-line with the input force and displacement or at an arbitrary angle with respect to input force and displacement.
Another advantage of the present invention is that the pivotless compliant mechanism can be designed to operate with a MEM actuator providing a range of displacement of less than or equal to 5 microns and generate therefrom an output displacement that is multiplied by a factor in the range of 5 to 60 (with a correspondingly reduced output force). This can allow, for example, the use of compact short-stroke electrostatic or thermal actuators to generate a range of displacement suitable for use in driving MEM ratcheting devices or microengines.
Yet another advantage of the present invention is that the pivotless compliant structure can be used in a reverse sense to multiply the force provided by a MEM actuator with a corresponding reduction in the range of displacement available from the compliant structure. This can allow, for example, the use of long-stroke electrostatic comb actuators to provide an increased force over that which could otherwise be generated.
These and other advantages of the present invention will become evident to those skilled in the art.
In achieving the above and other objects, the present invention is a pivotless compliant mechanism, also referred to herein as a displacement modifying structure, which can be utilized individually or with similar structures to form a compliant transmission device (CTD). The main building block of such a CTD pursuant to this invention is the base displacement modifying structure itself, which may be viewed as a triangular form or element with two of its legs formed by beams, the beams being connected to one another at one end, and having a third (imaginary) leg defined across the distance between the two opposing ends of the beams. The displacement modifying structure of the present invention comprises a static beam and dynamic beam. The static beam has a fixed end and a moveable end while the dynamic beam has a first end and a second end, with the second end being connected through a pivotless joint to the moveable end of the static beam. Upon movement by an actuator of the first end of the dynamic beam over a first distance, the connected second end of the dynamic beam and the moveable end of the static beam move over a second distance. This second distance is modified relative to the first distance.
While the term xe2x80x98modifiedxe2x80x99 has been thus far used in describing the present invention, it should be understood that xe2x80x98amplifyingxe2x80x99, xe2x80x98attenuatingxe2x80x99 and xe2x80x98reorientingxe2x80x99 are intended to be interchangeable therewith unless otherwise noted. Hereafter, the word xe2x80x98amplifyingxe2x80x99 will be used in describing the invention since it is believed that the invention""s biggest field of use will be in amplifying displacement.
When properly designed, a singular triangular element of the present invention amplifies the motion from the input force and orients it in a direction generally perpendicular to the triangle""s apex (where the beams are joined together). In constructing a CTD, such triangular elements can be cascaded together in many different ways to achieve a desired displacement amplification. Such cascading or joining of a series of the displacement amplifying structures will be more clearly understood from the detailed discussion that follows. In some preferred embodiments, the amplified displacement or output is provided as linear motion and in others it is provided as rotary motion. In another embodiment, a series of displacement amplifying structures are arranged in mirror symmetry about a common axis to provide motion substantially along the axis.
The present invention, as applied in a MEM system, is formed on a substrate for receiving an input at one end (from an actuator) and generating a multiplied displacement which is provided to a load. The structure of the MEM system comprise a flexible beam, the static beams, fixedly attached at one end to the substrate, with the other end of the static beam being moveable. A dynamic beam is connected between the actuator and the moveable end of the static beam. A series of additional static and dynamic beams, with each dynamic beam connected between the moveable ends of successive static beams, completes the structure. The structure can be formed using surface micromachining processes, with the structure preferably being formed from polycrystalline silicon, or alternately from silicon nitride. The structure can be used in combination with an actuator or formed on the same substrate (i.e. a silicon substrate) with the motive source, which may be a MEM actuator such as an electrostatic comb actuator, a capacitive-plate electrostatic actuator, or a thermal actuator.
According to various embodiments of the present invention, input and output can either move substantially in-phase with respect to each other or substantially out-of-phase with respect to each other for operation below resonance. Furthermore, the output displacement can be directed either substantially in-line with the first force of actuation or at an arbitrary angle with respect to the first force of actuation. Finally, the output displacement range can be either greater than the first linear displacement range (e.g. a multiplication factor of 5-60 when the first linear displacement range is 5 xcexcm or less), with the second force of actuation being correspondingly smaller than the first force of actuation; or alternately, the second linear displacement range can be less than the first linear displacement range (e.g. about one-fifth to one-sixtieth of the first linear displacement range), with the second force of actuation being correspondingly greater than the first force of actuation (e.g. by a factor of 5-60).
In another embodiment, the displacement modifying structure is operatively coupled to at least one other displacement modifying structure thereby forming a displacement-modifying device.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.